SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0645 AM CST Mon Nov 30 2020 Valid 301300Z - 011200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM EASTERN NORTH CAROLINA TO NEW JERSEY... ...SUMMARY... At least isolated damaging winds and a couple of tornadoes will be possible today from parts of Florida into the Carolinas, Mid-Atlantic and southern New England. ...Synopsis... A progressive, rather highly amplified synoptic pattern will characterize the mid/upper troposphere over the CONUS. The most important feature will be a strengthening trough -- now apparent in moisture-channel imagery from Lake Superior across MO to east TX. A large, closed cyclone should develop along the trough, over the east-central CONUS, between 00-06Z. Between 00-12Z, the corresponding 500-mb low should move northeastward up the Appalachians from eastern TN to western PA. A shortwave trough -- initially over the southern Appalachians -- will accelerate northeastward and weaken today, reaching western NY by 00Z. The 11Z surface analysis showed the associated synoptic cyclone over the western NC/southwestern VA/northeastern TN area, with a couple embedded lows. One was a frontal-wave low in the CLT/HKY area that should become the primary cyclone center with time as it ripples northeastward. A cold front was drawn from that low across central SC, southeastern GA, northwestern FL, and the eastern through south-central Gulf. The low should reach central NY by 00Z, then decelerate and move erratically near Lake Ontario or northern NY through tonight as it occludes more deeply. At that time, the cold front should reach the coastal Mid-Atlantic, then offshore of the Carolinas to southeastern FL. The warm front will move northward through the Mid-Atlantic to southern New England today. ...Mid-Atlantic to southern New England... Scattered thunderstorms are expected to continue on both sides of the warm front, within a large mass of convection and precip, becoming isolated to widely scattered southwestward across the warm sector to the cold front. As this regime shifts northeastward through the Carolinas and Mid-Atlantic to southern New England today through this evening, sporadic damaging to severe gusts and a few tornadoes are possible. A combination of subtle diurnal heating (beneath abundant cloud cover) and boundary-layer theta-e advection will contribute to enough destabilization to offset poor midlevel lapse rates, yielding a northeastward-shifting corridor of 500-1000 J/kg MLCAPE over the "slight" area. This buoyancy will diminish northeastward into southern New England along and south of the warm front where theta-e will be weaker. A shallow stable layer may persist near the surface in northern fringes of the "marginal" area, but with potential for momentum transfer in the strongest convective downdrafts to penetrate that at strong-severe levels. As the ejecting shortwave passes by to the west, the low-level cyclone deepens, and the synoptic trough approaches, deep-layer flow will strengthen region-wide. 45-60-kt effective-shear vectors should develop over the warm sector, where a 40-65-kt LLJ also should develop. The later will lead to the potential for somewhat curved and lengthy hodographs supporting supercells and small bows, in a low-LCL setting, supporting some tornado potential in addition to strong-severe gusts. ...FL... Widely scattered to scattered thunderstorms are evident in a pre-frontal convective band from northeastern FL to near SRQ. Though isolated convection is possible to its southeast, this primary convergence line should be the main convective focus through this afternoon as it crosses the peninsula and reaches at least the lower Keys. Isolated damaging to severe gusts are possible. The front will encounter a progressively more-unstable boundary layer in central/southwestern FL, and existing rich moisture from there toward the lower Keys, related to a reservoir of maximized theta-e now analyzed to their west. Preconvective veering of surface winds is forecast, which will be involved with some counterbalancing effects: 1. Reducing low-level and deep/speed shear, though the latter still may be sufficient to support isolated organized convection, with strong (50-60-kt) 500-mb flow over the surface boundary. 2. Reducing of kinematic contribution to low-level convergence/lift, though diurnal surface destabilization -- and related weakening of MLCINH -- will help to maintain some convective-scale lift as convergence gradually lessens. 3. Increase in boundary-layer moisture as the increasing westerly component advects Gulf parcels into the region and erodes an axis of antecedent min theta-e. ..Edwards/Broyles.. 11/30/2020
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